Natural gas is a clean, safe and convenient fuel and has attracted much attention as an alternative energy to solid fuels such as oil, coal, etc. Moreover, the use of natural gas has continuously increased in various fields such as domestic, commercial, transport, and industrial uses and has become the foundation of the energy industry as an energy source that supplies about one fourth of the world's energy consumption together with solid fuels such as oil, coal, etc.
When the natural gas is extracted from a gas field and transported in a gaseous state, the volume is huge, and it may explode. Thus, to solve these problems, a method of cooling the natural gas to its liquefaction temperature to produce a liquefied natural gas (LNG) and storing and transporting the produced LNG in an LNG tank on an LNG carrier is mainly used. Typically, the liquefied natural gas contains about 600 times as much as natural gas per unit volume.
However, the liquefaction of methane gas as a main component of liquefied natural gas requires an extremely low temperature of about −162° C., which is very costly to manufacture a natural gas carrier for sea and land transport as well as a liquefied natural gas production facility.
Another method for storing and transporting natural gas is to use a compressed gas. However, the production of large vessels is technically difficult and requires a high cost due to high storage pressure, and it has a safety problem due to high pressure explosion.
On the contrary, natural gas hydrate provides about 170 times as much as gas per unit volume and is produced at relatively moderate pressure and temperature (at 40 bar and 3° C.). Once the gas hydrate is formed, the preservation of gas hydrate is made at −20° C. and 1 atmospheric pressure. These temperature and pressure conditions of natural gas hydrate are more moderate than those of liquefied natural gas and compressed gas.
Moreover, even when the natural gas hydrate is exposed to room temperature and atmospheric pressure, it is less likely to explode, and thus it is possible to ensure sufficient time to cope with any leakage or damage of the system, thus ensuring safety. That is, the storage and transportation of natural gas hydrate are safer and more economical than those of liquefied natural gas (LNG) or compressed natural gas (CNG).
The natural gas hydrate is a compound in the form of dry ice formed by the physical combination of gas and water at low temperature and high pressure rather than the chemical combination. The calorific value of 1 m3 of gas hydrate is the same as about 180 m3 of natural gas. Naturally, the natural gas hydrate is found as a crystal, in which gas and water are combined, in the submarine or frozen earth where the temperature is low and the pressure is high. Moreover, the natural gas hydrate is easily decomposed into gas and water under dissociation conditions.
The natural gas hydrate can be classified into I-type, II-type, H-type, etc. according to the molecular structure. The natural gas hydrate is similar to ice in appearance but has a structure different from that of ice. While ice has a two-dimensional planar structure at a low temperature near 0° C., when the natural gas hydrate is maintained at an appropriate pressure (20 to 40 bar), a water molecule forms a three-dimensional cavity structure.
Assuming a spherical body, the size of a single cavity is about 1 nanometer, the unit cell size is about 2 nanometers, and the natural gas flows into the cavity. That is, water molecules connected by hydrogen bonds become a “host”, and gas molecules become a “guest”. The general formula of gas hydrate is Gas(H2O)n, where n denotes the hydration number and has about 5 to 8 depending on the size is of the gas molecule. The van der Waals force acts between nonpolar gas molecules and water molecules.
A typical method for forming a natural gas hydrate is a bubbling method in which a high pressure cooled natural gas supplied through a gas nozzle installed at the top of a reactor is in contact with water injected through a nozzle installed below the gas nozzle or through a porous plate installed at the bottom of the reactor, thus forming a natural gas hydrate. According to this method, the overall reaction is an exothermic reaction, and thus a cooling system is installed in the reactor to remove heat generated during the reaction or a system for lowering the temperature of the reactor is provided on the outside of the reactor.
However, according to this method, the formed natural gas hydrates may cause plugging in a raw water or natural gas injection nozzle, and when an injection plate is used, the mass transfer resistance increases during the formation reaction due to the large diameter of the formed raw water particles, which is problematic. Moreover, it is difficult to separate the formed natural gas hydrates from unreacted water, and the amount of unreacted water increases due to low conversion rate, which in turn increases the amount of energy required for separation and reuse processes.
Moreover, existing methods for forming natural gas hydrates have many problems in industrialization due to long induction time for hydrates and low hydrate crystal growth rate. Here, the induction time for hydrates may be defined as a period during which the hydrate is maintained in a metastable liquid state until solid gas hydrate crystals are formed, and the induction time for methane hydrates is generally several days.
Recently, Rogers and Zhong have reported that the induction time for ethane hydrates was reduced to about 40 minutes when a surfactant of sodium dodecyl sulfate (SDS) and a double-cooling system in the inside and outside of a reactor were used (2002). Here, the overall production rate of ethane hydrates was increased about 700 times compared to the non-use of SDS.
However, the crystal nucleation (hydrate crystal induction) is probabilistic, and the fast induction does not occur consistently and repeatedly even under the same experimental conditions and the same surfactant concentration and is affected by various factors such as impurities in an aqueous solution, external vibration, cooling method, etc.
Thus, there is an urgent need for the development of a new method for forming a gas hydrate, which can solve the above-described problems associated with the existing methods for forming gas hydrates and increase the gas hydrate crystal growth rate without any induction time for gas hydrates, in the utilization of natural gas energy.